In a conventional radiation pyrometer, part of the thermal radiation emitted by a hot body is focused onto a thermopile where the resultant heating of the thermopile causes an electrical signal to be generated in proportion to the thermal radiation. The signal thus generated may then be calibrated for display and recording of temperature. The problem is that thermal radiation from the body depends not only on its surface temperature but also its surface characteristics.
Pyrometers are classified generally into two types. One type referred to as a total-radiation pyrometer requires that the field of view be filled and the other type does not. In the one type, a comparison is made between the radiation of the body whose temperature is to be measured and the radiation of a reference body. For an example of that one type, known as an optical pyrometer, an image of the body is focused in the plane of a filament that is heated by current the amplitude of which is controlled by a calibrated rheostat. The filament can be readily viewed in that plane until the controlled current heats the filament to a temperature at which its radiation blends into the image of the body. Temperature of the body is then read from the calibrated rheostat. However, the accuracy of such an optical pyrometer will still depend upon its surface emissivity.
One approach for the implementation of an optical pyrometer of the second type, which does not require the field of view to be filled, is to make measurements of the radiation emitted by the body in two wavelength (color) regions. A calibrated ratio of the two measurements is then accepted as the true temperature of the body. The assumption is that the effect of surface characteristics cancels out in the ratio measurement sufficiently to provide a measurement close to the true temperature.
Yet another approach for implementation of a pyrometer of the second type assumes that thermal radiation from a black body falls on a sample body whose temperature is to be measured at a convenient angle of incidence and that a radiation detector measures total reflected and emitted radiation. The true temperature of the sample body is then determined from the calibrated measurement of radiation provided the black body and the sample body are maintained at the same temperature. In that special case, the total thermal radiation (emitted and reflected) detected from the sample body is the same as if the radiation detector received emitted radiation from only the black body which is at the same temperature as the sample body. However, that assumption is believed to be erroneous because of random scattering of radiation from the black body by the sample body due to its surface characteristics. See the background discussion of U.S. Pat. No. 3,442,673 titled APPARATUS AND METHOD FOR MEASURING TEMPERATURES by Thomas P. Murray.
To overcome that erroneous assumption, Murray discloses a system which uses a rotating polarization analyzer 52 which generates from the combined reflected and emitted radiation a cyclical resultant beam. Then successive measurements of the cyclical resultant beam are made while the black body temperature is varied until a null measurement is reached. That null identifies when the black body temperature equals the sample body temperature. While this use of a rotating polarization analyzer overcomes what is believed to be an erroneous assumption, the apparatus is complex and cumbersome to use not only because of using a rotating polarization analyzer but also because of using a controlled black body reference.
An alternative approach for the implementation of an optical pyrometer of the second type is disclosed in U.S. Pat. No. 3,462,224 titled POLARIZATION PYROMETER by W. W. Woods et al. There a laser 11 replaces the black body source of radiation incident at a convenient angle. Reflected radiation from the laser is first measured together with thermal radiation from the reflecting surface through a polarization state analyzer (PSA) alternately set for measuring the component of polarization p in the plane of incident radiation from the laser and the polarization component s perpendicular to that plane. The difference between measured p and s radiation is taken by a difference amplifier 26 and used to adjust the laser intensity until that difference is nulled. After that laser intensity adjustment, the radiation emanating from test surface 10 corresponds to that of a black body at the unknown temperature of the test body. Then the p and s components of reflected radiation from a standard lamp 20 are similarly measured using a chopper 19 to switch the polarizer and detector from the reflected laser radiation to a standard lamp and a difference amplifier 30 is used to measure the difference between the p and s components of light reflected this time by a member 19a of the chopper 19. That second measured difference is used to adjust the intensity of the standard lamp until the second measured difference is nulled. The radiation from the lamp is then assumed to also correspond to radiation from a black body at the temperature of the test surface (Col. 4, lines 176 to 26). By alternately switching the chopper 19 between laser light reflected by the test surface and reference light reflected by the chopper member 19a and automatically adjusting the laser and reference light to bring the system to a stable condition, the energization of the standard lamp 20 is assumed to be proportional to the temperature of the test surface 10. That assumption is believed to be inaccurate.
Another example of an optical pyrometer which determines the temperature of a body from its thermal radiation and seeks to overcome the effects of surface characteristics of the body is disclosed in U.S. Pat. No. 5,011,295 titled METHOD AND APPARATUS TO SIMULTANEOUSLY MEASURE EMISSIVITY AND THERMODYNAMIC TEMPERATURE OF REMOTE OBJECTS by S. Krishnan et al. That system is even more complex than the two discussed above. It seeks to separately determine emissivity, reflectivity and optical constants as well as the apparent brightness temperature of the sample with a single instrument in order to correct the brightness temperature of the sample and thus provide a true temperature free of effects due to surface characteristics.